DNA binding is rate-limiting for natural transformation.

Bacteria take up environmental DNA using dynamic appendages called type IV pili (T4P) to elicit horizontal gene transfer in a process called natural transformation. Natural transformation is widespread amongst bacteria yet determining how different factors universally contribute to or limit this process across species has remained challenging. Here we show that the most naturally transformable species, is highly transformable due to its ability to robustly bind nonspecific DNA via a dedicated orphan minor pilin, FimT. We show that, compared to its homologues, FimT contains multiple positively charged residues that additively promote DNA binding efficiency. Expression of FimT in a closely related pathogen is sufficient to substantially improve its capacity for natural transformation, demonstrating that T4P-DNA binding is a rate-limiting step in this process. These results demonstrate the importance of T4P-DNA binding efficiency in driving natural transformation, establishing a key factor limiting horizontal gene transfer.